1. Field of the Invention
The present invention relates to power converter control apparatus intended for converting direct-current power into alternating-current power and driving an induction motor.
2. Description of Prior Art
In the railroad rolling stock field, the induction machines controlled by the power converters for converting direct-current (hereinafter referred to as DC) trolley wire voltages into variable-frequency variable alternating-current (likewise, referred to as AC) voltages are used to drive electric rolling stock. An induction machine control method using prior art is shown in FIG. 2. In FIG. 2, numeral 1 denotes a current collector; numeral 2, a filter reactor; numeral 3, a filter capacitor; numeral 4, a power converter; numeral 5, an induction motor; numeral 6, a rotational speed detector; numeral 7, a current detector; numeral 8, a bandpass filter; numerals 9 and 10, adders; numeral 11, an integrator; numeral 12, a coefficient multiplier; and numeral 13, a voltage command unit.
The DC voltage from the trolley wire is received at current collector 1, then the high-frequency components of the received voltage are removed via filter reactor 2 and filter capacitor 3, and the DC voltage is supplied to power converter 4. Power converter 4 converts the DC voltage into a variable-frequency variable AC voltage in accordance with AC voltage commands Vu*, Vv*, and Vw*, and controls induction machine 5. The slip frequency command xcfx89s* externally set to the rotational speed xcfx89r which has been detected by rotational speed detector 6 provided to detect the rotational speed of induction machine 5 is added by adder 9, and thus a first frequency command value xcfx8913* is calculated. After being detected by current detector 7, the trolley wire current Is flowing through filter reactor 2 is sent to bandpass filter 8, and components in the neighborhood of the resonance frequency determined by the capacities of filter reactor 2 and filter capacitor 3 are detected. The frequency correction value dxcfx89 that has been sent from bandpass filter 8, and the first frequency command value xcfx8913* mentioned above are added by adder 10, and then a second frequency command value xcfx8914* is calculated. The second frequency command value xcfx8914* is integrated by integrator 11 and thus the voltage phase xcex8v is obtained. Also, coefficient multiplier 12 multiplies the first frequency command value xcfx8913* by a predetermined frequency voltage ratio K, and thus a voltage command value V* is calculated. Voltage command unit 13 issues AC voltage commands Vu*, Vv*, and Vw* in accordance with the above-mentioned voltage phase xcex8v.
The control method described above is disclosed in Japanese Laid-open Patent Application No. Sho 57-145503 (JP A 57-145503). A method of controlling AC voltages, based on trolley wire currents, is also disclosed in Japanese Laid-open Patent Application No. Sho 57-145503 (JP A 57-145503). According to the above prior art, the instability and vibration that are caused by the resonance of the filter reactor and the filter capacitor can be suppressed.
Also, methods of controlling electric rolling stock by use of an induction machine are disclosed in Japanese Laid-open Patent Application No. Hei 05-83976 and 2000-312403 (JP A 05-83976 and JP A 2000-312403).
In railroad rolling stock, the return wire current of the DC voltage which has been supplied from the trolley wire to the power converter is induced into the track along which electric rolling stock travels. The track with safety devices installed on the ground is also used as a track circuit for detecting the electric rolling stock on the line. Accordingly, faults may occur in the track circuit if the return wire current of the power converter includes a fault current of the same frequency components as those of the AC signal used for the track circuit. In this case, therefore, the fault current within the return wire current needs to be removed. The return wire current flowing into the track and the trolley wire current flowing into the filter reactor are of the same value. For this reason, in the description given below, both currents are unified into a trolley wire current, except when their distinction is necessary.
Although the fault current included in the trolley wire current can be removed using the filter reactor and filter capacitor mounted in the railroad rolling stock, the inductance of the filter reactor and the capacity of the filter capacitor need to be increased in order to achieve applicability to a track circuit which, as with a divided-and-doubled frequency track circuit, uses a low frequency. If the above factors are increased, however, this will also increase the weight of the railroad rolling stock, thus resulting in problems such as deteriorated acceleration/deceleration performance and increased power consumption.
Under prior art, controlling the current at the DC side of the power converter is accomplished by adjusting the frequency according to the particular trolley wire current and controlling the electric power of the induction machine, and thus the fault current is eliminated. However, when the frequency is adjusted and the electric power of the induction machine is controlled, the electrical delay factors of the induction machine will cause a significant time lag in electric power with respect to the amount of frequency adjustment (hereinafter, referred to as the manipulated variable). FIGS. 3(a) and (b) show the frequency characteristics that cover the range from the manipulated variable to the electric power of the induction machine, and six frequency response curves are shown that were obtained when the rotational speed of induction machine 5 was changed. The horizontal axis in FIG. 3(a) denotes the frequency of the AC components included in the manipulated variable, and the vertical axis denotes the amplitude ratio between the frequency components shown in the horizontal axis and the corresponding AC components of the electric power of the induction machine. The horizontal axis in FIG. 3(b) denotes the same as in FIG. 3(a), and the horizontal axis denotes the differences in phase between the corresponding AC components and the electric power of the induction machine. It can be seen from FIGS. 3(a) and (b) that in the 25-60 Hz band commonly used for track circuits, the phase of the electric power of the induction machine significantly delays behind the manipulated variable. If such feedback loop as shown in FIG. 2 is formed in the case that there is a significant phase delay in the range from the manipulated variable to the controlled variable, operation will tend to become unstable, and if the gain of the feedback loop is increased for suppressed fault current, the control system will become unstable and the fault current included in the trolley wire current will not be effectively controllable.
Even with a control method in which the AC voltage is controlled in accordance with the trolley wire current value, the fault current cannot be effectively suppressed since the electric power of the induction machine is significant in phase delay behind the manipulated variable.
An object of the present invention is to suppress effectively the fault current included in the trolley wire current, without increasing the capacities of the filter reactor or the filter capacitor.
The above object can be realized by using power converter control apparatus that can
issue an AC voltage command, subject to a voltage command and a frequency command, to a power converter for converting a DC voltage into a variable AC voltage of a variable frequency,
have a detector for detecting the DC voltage of the power converter, namely, the amount of electricity generated at the direct-current side of the power converter, or for detecting the voltage developed across a filter reactor, or for detecting the current flowing through the filter reactor,
a high-pass filter for removing direct-current components from the detected amount of electricity, a voltage corrector for correcting said voltage command in accordance with an output signal from said high-pass filter, and
a frequency corrector for correcting said frequency command in accordance with an output signal from said high-pass filter,
and send an AC voltage command in accordance with output signals from said frequency corrector and said voltage corrector.
Since internal interference of an induction machine connected to the AC side of the power converter can be suppressed by simultaneously controlling frequency and voltage in the way described above, the electric power of the induction machine can be controlled with an insignificant phase delay and the fault current can be suppressed effectively.
Also, even if the voltage at the AC side of the power converter saturates, stable control can be implemented by stopping the voltage command operations and changing the control mode to the control of the frequency command only.